Stent

ABSTRACT

A stent, especially a coronary stent, comprising at least one thin-walled, tubular member with an openly reticular outer surface containing recesses, which are defined by narrow weblike members. Weblike members are formed from the remaining material from the tube wall once the material has been removed from the recess area. The weblike members are shaped in such a way as to substantially reduce the deformation to which a weblike member is subjected between connecting areas with other weblike members during expansion. The invention also relates to a method of producing the like.

The invention relates to a stent, particularly a coronary stent, as anintraluminal expansion element of the type mentioned in the generic partof claim 1, and to methods of making such a stent.

An expandable intraluminal element with at least one thin-walled,tubular member (hereinafter referred to as a stent) is known fromEuropean patents EP-B1 0 364 787 and EP-B1 335 341. The generatedsurface of the stent is in the form of an open network and has aperturesbounded by strap-like elements of low material thickness, extending in astraight line in the axial and peripheral directions. The strap-likeelements comprise the remaining wall of the tube, from which thematerial in the area of the apertures has been removed.

During an operation such stents are expanded by the action of outwardlydirected forces, by means of a tubular dilator working with compressedgas. The stent retains its tubular shape in spite of deformation andexpands the vessel which has been narrrowed by deposits.

The known stent has the drawback that expansion through deformation ofthe axially extending strap-like elements can only take place to alimited extent, as relatively narrow limits are set to a change in theshape of the individual strap-like elements of the stent. These limitsdepend on the material tensions which accompany deformation, and whichmay lead to breakage of one or more of the strap-like elements formingthe network if the deformation becomes too great.

For safety reasons therefore deformation must normally be kept far belowa possible danger range, as breakage of a strap would cause its freeends in the vicinity of the breakage point to project into the interiorof the vessel provided with the stent. The concomitant danger ofrestenosis formation would not only put the success of the operationinto question but also endanger the patient's life.

Based on the defects of prior art, the problem of the invention is toprovide an expandable stent of the above type, which can be expanded assafely as possible--and thus without any risk of breakage in thevicinity of the strap-like element through tensile overloading.Shortening of the stent must also be avoided.

The problem is solved by the characterising features of claim 1.

The invention includes the technical teaching that, in the case offragile tubular elements which are made of suitable materials, whichhave network-like structures and which are subject to deformation inuse, critical loads on the material or even breakages of the materialcan be avoided, if the maximum tensions occurring in areas subject toincreased deformation are limited from the outset by the design.Shortening of the stent during its expansion may also be avoided if, inaddition, pairs of transversely expandable elements are directly joinedat the ends longitudinally of the stent, each of the two expandableelements, in the form of a flattened ring element made up of straps,being joined transversely by a strap-like element to anothertransversely expandable element, which is itself not directly joined atthe end to another expandable element, which is in turn joined by astrap-like element to one of the first-mentioned, transverselyexpandable elements, the strap-like elements each being inclined to thetransverse direction at an angle such that this angle is reduced onexpansion of the stent, and the non-joined adjacent ends of transverselyexpandable elements thus move away from each other.

This applies particularly if the strap-like elements in the non-expandedstent are at an inclination of substantially 45° to the transversedirection.

Through the process of stretching the expandable elements transverselyof the stent, which occurs simultaneously with the expansion of thestent, and through the alignment of the inclined joining elements in thetransverse direction, the non-joined groups of expandable elements aredisplaced relative to each other, in such a way that that movementcompensates for the shortening of the stent by extending the flat shapesof the expandable elements into O rings.

The transversely expandable elements interact particularly favourablywith the strap-like elements if the latter lead into the transverselyexpandable elements at an angle of less than 45°--relative to theirlocal direction.

If the strap-like elements are curved so as to lead into the strapregions of the expandable elements perpendicularly in the centre, aparticularly favourable input of forces is obtained.

Owing to the geometry of the arrangement in the non-expanded condition,it is beneficial for the rounding of the transversely expandableelements, in the end region where they are joined at the end to the nextsimilar element in the longitudinal direction, to have a larger radiusthan at the end where they are not joined.

If the transversely expandable elements have an ellipse-like orcircle-like shape in the expanded condition of the stent, the input offorce into the expanded vessel is particularly harmonious.

This also applies if the strap-like elements substantially act onregions of the transversely expandable element which are opposite eachother in the transverse direction--and particularly if they act on themsubstantially centrally--when the stent is expanded.

A particularly beneficial construction can be obtained, not merely bydesigning for the greatest possible strength by enlarging thecross-sections of the material, but also by optimising the shape of thestraps and joining regions with a view to the expected loads. This canbe done, on the one hand by locally minimising the maximum tensionsoccurring, but on the other hand by also controlling the necessarydeformations.

These relationships are described in a patent application filed at thesame time by the same Applicant.

As an advantageous precondition for the design it has been found thatdeformation is assisted if the shape of the stent in the non-expandedcondition substantially corresponds to the shape obtained when a sample,which has strap structures of regular shape in its expanded shape andwhich is made in that form from a tubular structure, is compressed intothe non-expanded shape, the eventual starting shape. Thus the formproduced as the starting shape corresponds to that obtained bycompressing a stent which was made in the expanded form.

Such regular shapes are preferably produced from circles, ellipses,rectangles, squares, polygons or structures combining these and/orapproximating to them.

To encourage local tension-free deformations, branches from straps areformed so as to avoid abrupt changes in the strap width. The appearanceof stress concentrations or notch tensions and the like is thus avoided.In this way material tensions, particularly in the region ofintersections or branches, can be prevented from exceeding apredetermined value on deformation, even as a notch tension.

A construction of this type gives the branches of strap-like elements aparticularly organic shape, substantially corresponding to the shape ofbranches from tree trunks.

In some cases it is beneficial for the strap-like elements forming linksbetween expandable elements to be substantially S-shaped, as areas ofmaximum deformation are then transposed from the lead-in or joiningareas to the free areas of strap.

It is further advantageous for areas of intersections also to be shapedso that a maximum change in the angle between adjacent strap-likeelements forming arms of a cross or branches is not exceeded for apredetermined expansion. This can preferably be done by givingstrap-like elements forming the arms of a cross or branches a curvedshape.

If the edges of strap-like elements have rounded portions in anintersecting or branching area or in the area of a joining regionlinking successive stent segments, so that sharp corners are avoided,the maximum tensions here will be limited.

In a preferred embodiment of the invention the expandable, substantiallyhollow cylindrical stent, with a generated surface which is given anetwork structure by apertures, has an organic conformation at thepoints joining the strap-like elements which bound the apertures, inorder to avoid a high notch tension there which might lead to breakage.Local breakage of the stent detrimentally produces free, relativelysharp-edged ends within the spatial configuration of the stent, whichmay on the one hand bore through the wall of the vessel, or on the otherhand reduce the free cross-section of the vessel by moving out into thebloodstream.

This conformation is characterised by a rounding of all points linkingthe strap-like elements which move relative to each other on expansionof the stent; it ensures, in a simple and at the same time advantageousmanner, that the amount of local deformation of the material at thepoints in the stent construction which are critical in respect of thetensile strain occurring when the stent is extended has a minimal value,through uniform distribution of the deformation work.

Coupling members in the form of cruciform, strap-like elements areprovided to join the individual strap-like elements extendingsubstantially axially and bounding the apertures; these membersinterconnect adjacent apertures in an axial direction, in each case attheir ends and in a tangential direction, always at the centre of theaxially extending strap-like elements.

In a preferred embodiment of the invention the arms of the cruciformcoupling members are curved and arranged so that they form the sides ofa substantially obtuse angle in the tangential direction and asubstantially acute angle in the axial direction. The strap-likeelements of the apertures, adjoining the arms of the coupling members,are shaped as an axially extended S, and the opposing strap-likeelements adjacent each other in a tangential direction are arrangedsymmetrically in mirror image.

In another embodiment of the invention with this configuration of thecoupling member, rounding of the material is advantageously provided atthe point of intersection, so that there is only a relatively slightchange in the angle subtended by the adjacent arms of the cruciformcoupling member on expansion of the stent. Consequently there is not ahigh notch tension at the intersection and in particular there are nolocal tension peaks, so that any risk of breakage on expansion isexcluded.

With the same end in view, the free sections of the apertures at theends of the stent, or at the ends of the stent segments in theabove-described embodiments of the invention, are curved so that onlyslight mechanical stresses arise in those areas too on expansion of thestent, and no extreme notch tension values occur.

The above-described construction of the surface of the stent accordingto the invention, with organic shapes, ensures substantially evendistribution of the deformation work done in the expansion process tothe respective aperture-bounding sections, and thereby avoids extremetensile stresses on individual points or areas on the stent surface.

To enable stenoses in curved blood vessels to be successfully treated byexpanding stents, the stent in another favourable embodiment of theinvention is divided into substantially identically shaped segmentsarranged in rows in an axial direction.

A preferred stent designed in the above manner is made of tantalum asthe material, and is provided with a coating of amorphous siliconcarbide.

Other advantageous further embodiments of the invention arecharacterised in the sub-claims and will now be explained in greaterdetail with reference to the accompanying drawings, together with thedescription of the preferred embodiment.

In the drawings:

FIG. 1 represents a development of the non-expanded stent structure asthe preferred embodiment, shown as a development of the invention inside elevation,

FIG. 2 shows the stent structure from FIG. 1 in the expanded condition,and

FIG. 3 shows the conditions in FIGS. 1 and 2 superimposed forcomparison.

A stent 1 reproduced in FIG. 1 basically has a tubular/hollowcylindrical shape with numerous openings surrounded by expandablestructural elements in the form of flattened rings. These willhereinafter be termed "expandable elements", and an example of them ismarked with a dash-and-dot line 2. The expandable elements 2 are formedby narrow strap-like regions 4 of square cross-section running roundthem, and are distinguished by the fact that they enclose an aperture 3in an annular shape. The elements 2, in the fully expanded condition inthis case, have almost the shape of a circle or an ellipse. It will beseen that the shape in the non-expanded condition is derived from thatin the expanded condition, although the stent never assumed thatcondition in its production. The shape was discovered in a simulatedprocess--based on an ideal shape to be adopted in the expandedcondition--with compression simulated in a mathematical model. As thematerial sets up uniform resistance to mechanical forces acting on itwhen compression is applied, local deformations are also evened out.Uniform arcs with maximal radii are formed rather than localised sharpbends. The resultant shape forms the basis for modelling thenon-expanded one, which can thus be converted to the required finalshape in the reverse direction with uniform local deformation.

The strap-like regions 4 surrounding an expandable element arestructured with a multiple S-like sweep. They each surround an aperture3, in such a way that tangentially adjacent, opposing, strap-likeregions 4 of the same or an adjacent aperture 3 are arrangedsymmetrically in mirror image. The ends of the expandable elements 2 inthe longitudinal direction have free arcuate regions 7 and 8 which aremore curved.

The expandable elements are shaped so that, when the stent has beeninserted in a vessel, they can be converted to a ring shape with minimumdeformation by dilation with a balloon catheter The arc 8 can be seen toassume a maximal radius. The opposing S-shaped arcs enable it to incurminimum deformation when expanded.

Joining regions 5 are provided between apertures 3 arranged adjacenteach other in axial (longitudinal) and tangential (transverse)directions on the generated surface of the stent 1, and mechanicallycouple the respective expandable regions 2. Rounding of the material isprovided in the crossing, joining region 5, so that the region has anorganic shape, reducing the occurrence of increased notch tensions.

The individual strap-like elements can be seen to be shaped so that thebending deformation to which such an element is subjected on expansionwithin the hollow cylindrical, tubular shape--resulting from theintegral of the local changes of angle on deformation, determined overthe length of the respective element between adjoining regions linkingit with other strap-like elements--is distributed over the length of theelement so that a predetermined strain on the material is not exceededeven locally.

The shape of the stent in the non-expanded condition substantiallycorresponds to the shape obtained when a sample, which has regularlyshaped strap structures in its expanded shape and which is made in thatform from a tubular structure, is compressed into the non-expandedshape--the eventual starting shape. The regular shape comprises circles,ellipses, rectangles, squares, polygons or structures combining theseand/or approximating to them.

Branches of straps are formed so as to avoid abrupt changes in the widthof the strap and so that tensions on the material, particularly in theregion of the branching, do not exceed a predetermined value ondeformation even as a notch tension. The stent represented in FIG. 1a isof substantially homogeneous structure over its whole length.

In the embodiment illustrated it will be seen that pairs of transverselyexpandable elements are directly joined at the ends longitudinally ofthe stent, each of the so expandable elements, in the form of aflattened ring element made up of straps, being joined transversely by astrap-like element to another transversely expandable element; thelatter element is itself not directly joined at the end to anotherexpandable element, which is in turn joined by a strap-like element toone of the first-mentioned, transversely expandable elements; thestrap-like elements are each inclined to the transverse direction at anangle which is reduced on expansion of the stent, and the non-joinedadjacent ends of transversely expandable elements thus move away fromeach other, so that shortening of the stent during its expansion can beavoided.

In the non-expanded stent the strap-like elements are inclined to thetransverse direction at substantially 45°.

Through the process whereby the expandable elements are stretchedtransversely of the stent simultaneously with the expansion of thestent, and through the alignment of the inclined joining elements in thetransverse direction, the non-joined groups of expandable elements aredisplaced relative to each other so that their movement compensates forthe shortening of the stent with the stretching of the flat shapes ofthe expandable elements into O rings.

FIG. 2 uses the same references as FIG. 1; it will be seen from it howthe FIG. 1 structure is transformed in the expanded condition to astructure with substantially round rings linked by straps. The joiningstraps between the expandable elements are now aligned substantiallytangentially.

In addition, the individual strap-like elements can be seen to be shapedso that the bending deformation to which an element is subjected onexpansion within the hollow cylindrical, tubular shape--resulting fromthe integral of the local changes of angle on deformation, determinedover the length of the respective strap-like element between adjoiningregions linking it with other strap-like elements--is distributed overthe length of the element so that it does not exceed a predeterminedvalue locally.

It will also be seen that branches of straps are formed so as to avoidabrupt changes in the width of the strap and so that tensions on thematerial, particularly in the region of the branch, do not exceed apredetermined value on deformation, even as notch tensions.

The constriction of the stent 1 described above allows the tubular stentto expand without the notch tension reaching extreme values at thejoints, leading to the breaking of strap regions.

As will be seen from FIG. 2 the maximum local deformations--and thus thedanger of extreme notch tension values being reached on expansion of thesegment 2--remain extremely low. Instead of being concentrated onindividual points of individual apertures 3, particularly the tip of theexternal arcuate pieces 7, they extend over the whole region of theapertures 3 bounded by the S-shaped strap-like elements 4 and the arms5.1, 5.2, 5.3. 5.4 of the joining regions 5.

Above all it will be seen that an expandable element formed by strapelements is deformed with local deformations being limited as far aspossible. The arcuate regions in the non-expanded condition are chosenas large as possible, so that when the stent is expanded all parts ofthe strap-like element are as far as possible equally involved in there-shaping.

In FIG. 3 the structure of the non-expanded stent from FIG. 1 and theexpanded stent from FIG. 2 are superimposed; it will be seen not justthat no shortening of the stent occurs but that there is even a certainlengthening, although on further expansion this is compensated for bythe return of the stent to its original length. Its applicationvirtually free of complications is thus ensured. In the embodiment shownin this figure the strap-like joining element 11' is curved doubly in anS shape in order to minimise deformations.

The stent illustrated here is made of tantalum, titanium or anotherbiocompatible alloy, as a material giving good compatibility with thebody and excellent deformability. A micro-coating of amorphous siliconcarbide counteracts thrombus formation.

The forms taken by invention are not restricted to the preferredembodiments described above. There are in fact a number of favourableversions which make use of the illustrated solution in fundamentallydifferent ways.

I claim:
 1. A stent, particularly a coronary stent, comprising at leastone thin-walled, tubular element, the generated surface of which is inthe form of an open network and has apertures bounded by narrow,strap-like elements, the strap-like elements being formed from theremaining material of the tubular wall from which the material in thearea of the apertures was removed, characterised in that pairs oftransversely expandable elements are directly joined at the endslongitudinally of the stent, each of the two expandable elements, in theform of a flattened ring element made up of straps, being joinedtransversely by a strap-like element to another transversely expandableelement, which is itself not directly joined at the end to anotherexpandable element, which is in turn joined by a strap-like element toone of the first-mentioned, transversely expandable elements, thestrap-like elements each being inclined to the transverse direction atan angle such that this angle is reduced on expansion of the stent, andthe non-joined adjacent ends of transversely expandable elements thusmove away from each other.
 2. A stent according to claim 1,characterised in that when the stent is not expanded the strap-likeelements have an inclination of substantially 45° to the transversedirection.
 3. A stent according to claim 1, characterised in that thestrap-like elements lead into the transversely expandable elements at anangle of less than 45'.
 4. A stent according to claim 1, characterisedin that the strap-like elements are curved so as to lead inperpendicularly in the centre.
 5. A stent according to claim 1,characterised in that the rounding of the transversely expandableelements in the end region where they are joined at the end to the nextsimilar element in the longitudinal direction has a larger radius thanat the end where they are not joined.
 6. A stent according to claim 1,characterised in that in the expanded condition of the stent thetransversely expandable elements have an ellipse-like or circle-likeshape.
 7. A stent according to claim 1, characterised in that when thestent is expanded the strap-like elements substantially act on regionsof the transversely expandable element which are opposite each other inthe transverse direction.
 8. A stent according to claim 7, characterisedin that the strap-like elements exert their action substantiallycentrally.
 9. A stent according to claim 1, characterised in that thestrap-like elements are shaped so that the total deformation which astrap-like element undergoes between joining regions to other strap-likeelements in the expansion process is substantially minimised.
 10. Astent according to claim 1, characterised in that the individualstrap-like elements are shaped so that the bending deformation to whicha strap-like element is subjected on expansion within the hollowcylindrical, tubular shape, resulting from the integral of the localchanges of angle on deformation, determined over the length of therespective strap-like element between adjoining regions with otherstrap-like elements, is distributed over the length of the element sothat it does not exceed a predetermined value locally.
 11. A stentaccording to claim 1, characterised in that the bending deformationapplied to a length of one-fifth of a strap-like element is no greaterthan a quarter of the total bending deformation to which that element issubjected.
 12. A stent according to claim 1, characterised in that theshape of the stent in the non-expanded condition substantiallycorresponds to the shape obtained when a sample, which has strapstructures of regular shape in its expanded shape and which is made inthat form from a tubular structure, is compressed into the non-expandedshape, the eventual starting shape.
 13. A stent according to claim 12,characterised in that the regular shape comprises circles, ellipses,rectangles, squares, polygons or structures combining these and/orapproximating to them.
 14. A stent according to claim 1, characterisedin that branches from straps are formed avoiding abrupt changes in thestrap width.
 15. A stent according to claim 1, characterised in thatbranches are formed so that the material tensions, particularly in theregion of the branching, do not exceed a predetermined value ondeformation, even as a notch tension.
 16. A stent according to claim 1,characterised in that the strap-like elements (4, 11') are substantiallyS-shaped.
 17. A stent according to claim 1, characterised by titanium,tantalum or another biocompatible metal or a corresponding metal alloyas the material.
 18. A stent according to claim 17, characterised inthat a coating of amorphous silicon carbide is provided.